1. Field of the Invention
This invention relates to a scanning wide-area surface shape analyzer as measuring means incorporating an interferometer for scanning a surface to be inspected to calculate a surface shape thereof.
2. Prior Art
As a method of measuring the surface shape of a surface to be inspected having a generally planer shape with high accuracy, there has been proposed one in which a beam emitted from a light source is projected by an interferometer on a reference surface and a surface to be inspected having a generally planer shape to thereby generate an optical interference fringe image formed by interference between reflected light from the reference surface and reflected light from the surface to be inspected, and the surface shape of the surface to be inspected is measured by using the optical interference fringe image. According to this method using an interferometer, although it is possible to measure the surface shape of a surface to be inspected with high accuracy, if the surface to be inspected has an observation area having undulations of a few xcexcm or more in a vertical direction, the number of fringes in the area increases, thereby making it difficult to measure the surface to be inspected.
Generally, as the area of a surface to be inspected increases, the amount of vertical undulation thereof also increases. To measure the surface shape of a broad surface to be inspected having large vertical undulations, it is required to use a scanning-type apparatus incorporating a scanner for moving an interferometer relative to the observation area of the surface to be inspected. However, this scanning-type apparatus as well undergoes measurement constraints related to the amount of vertical undulation.
To use an interferometer for measuring the surface shape of a surface to be inspected with accuracy, it is preferable to hold the interferometer in an attitude in which the reference surface of the interferometer and the surface to be inspected are parallel or approximately parallel with each other. In other words, the number of interference fringes included in the obtained interference fringe image represents the attitude of the interferometer relative to the surface to be inspected for data acquisition therefrom, and it is thus desired that the number of interference fringes is equal to or smaller than the number corresponding to the required measurement accuracy.
One will contemplate that an apparatus for measuring the surface shape of a broad surface to be inspected by using an interferometer that can meet this desire should include a scanner for moving the interferometer-relative to the surface to be inspected, and an acquisition attitude-varying device which is capable of changing the attitude of the interferometer relative to the surface to be inspected such that the reference surface and the surface to be inspected are parallel or approximately parallel with each other. In this apparatus, when the surface shape of a broad surface to be inspected is measured, a preparatory measurement is carried out to obtain optical interference fringes the number of which is equal to or smaller than a predetermined number, and a control variable for the attitude control for obtaining optical interference fringes such that the number of fringes is made smaller than the predetermined number is calculated for each acquisition area for acquiring image data obtained by the preparatory measurement. Then, an actual measurement is carried out while the attitude of the interferometer is controlled by using the calculated control variable for the attitude control.
The above-mentioned apparatus, however, requires execution of not only a preparatory measurement of the same surface to be inspected for obtaining a equal or smaller number of interference fringes to or than a predetermined number but also to an actual measurement while controlling the attitude of the interferometer by using the control variable for the attitude control. Therefore, it takes an increased or doubled time period to measure the surface shape of the surface to be inspected
It is an object of the present invention to provide a scanning wide-area surface shape analyzer which is capable of drastically shortening a time period required for measuring the surface shape of a surface to be inspected.
To attain the above object, the present invention provides a scanning wide-area surface shape analyzer comprising an optical table having a flat surface for placing an object to be inspected thereon, measuring means having a light source, a reference surface, and an interferometer which projects a beam emitted from the light source onto the reference surface and an surface to be inspected of the object placed on the flat surface, thereby generating an optical interference fringe image formed by interference between reflected light from the reference surface and reflected light from the surface to be inspected, the measuring means acquiring the optical interference fringe image generated by the interferometer as measured image data, scanning means having a scanning reference position thereof set to a point of intersection between an optical axis of the beam and the reference surface in the measuring means, for moving the measuring means in parallel with the flat surface such that the scanning reference position sequentially reaches each acquisition position used for the measuring means to acquire the measured image data, attitude varying means for varying an acquisition attitude of the measuring means for acquiring the measured image data, such that a direction of inclination of a reference axis being a direction vector extending along the optical axis from the scanning reference position is changed relative to the surface to be inspected, surface shape analyzing and calculating means for analyzing and calculating, based on the measured image data acquired at the acquisition position and in the acquisition attitude, a surface shape of an acquisition area corresponding to the acquisition position and the acquisition attitude, in the surface to be inspected, and control means for controlling driving of the scanning means and the attitude-varying means, and the control means calculates a next acquisition position by using measured image data acquired at a present acquisition position and in a present acquisition attitude, and calculates an acquisition attitude to be taken at the calculated next acquisition position.
According to the above construction, as is distinct from the conventional analyzer, there is no need to execute a preparatory measurement for obtaining the control variable of the measuring means, which makes it possible to drastically shorten a time period required for measuring the surface shape of an surface to be inspected.
Preferably, the control means obtains a position corresponding to an end position of the acquisition area in the surface to be inspected on a scanning line extending from the present acquisition position in a direction of scanning of the measuring means, based on a surface shape of the acquisition area in the surface to be inspected, which has been determined from the measured image data acquired at the present acquisition position and in the present acquisition attitude, and the control means calculates the next acquisition position such that a distance from the present acquisition position to the next acquisition position becomes not more than two times as large as a distance from the present acquisition area to the obtained position corresponding to the end position of the acquisition area in the surface to be inspected.
By thus calculating the next acquisition position, an overlapping area is positively made to exist between a acquisition area corresponding to the present acquisition position and an acquisition area corresponding to the next acquisition position, thereby preventing occurrence of an unmeasured area on the surface to be inspected.
Preferably, the control means calculates a perpendicular vector at a point of intersection between a line drawn from the calculated next acquisition position toward the acquisition area in the surface to be inspected or toward an extended area of the acquisition area, and the acquisition area or the extended area of the acquisition area, the line extending parallel with the reference axis at the present acquisition position and in the present acquisition attitude, the acquisition area corresponding to the measured image data acquired at the present acquisition position and in the present acquisition attitude, and the control means sets a direction of inclination of the reference axis which becomes coincident or parallel with the calculated perpendicular vector, to the acquisition attitude at the next acquisition position.
By thus determining the acquisition attitude at the next acquisition position, it is possible to hold the measuring means in an acquisition attitude in which the reference surface of the interferometer in the measuring means and a corresponding acquisition area on the surface to be inspected are substantially parallel with each other.
More preferably, when the measuring means is moved to the next acquisition position, if a position to which the measuring means has been moved is slightly different from the calculated next acquisition position, the control means calculates a perpendicular vector at a point of intersection between a line drawn from the position to which the measuring means has been moved, toward the acquisition area in the surface to be inspected or toward an extended area of the acquisition area, and the acquisition area, the line extending parallel with the reference axis at the present acquisition position and in the present acquisition attitude, the acquisition area corresponding to the measured image data acquired at the present acquisition position and in the present acquisition attitude, and the control means sets a direction of inclination of the reference axis which becomes coincident or parallel with the calculated perpendicular vector, to the acquisition attitude at the next acquisition position.
By thus determining the acquisition attitude at the next acquisition position, if the measuring means is moved to a position slightly different from the calculated next acquisition position, it is possible to correct the acquisition attitude of the measuring means.
Preferably, the control means determines a point of intersection between a line drawn from the calculated next acquisition position toward the acquisition area in the surface to be inspected and the acquisition area, the acquisition area corresponding to the measured image data acquired at the present acquisition position and in the present acquisition attitude, the line extending parallel with the reference axis at the present acquisition position and in the present acquisition attitude, calculates a least squares approximate surface based on the determined point of intersection and points close thereto, and sets a direction of inclination of the reference axis which becomes coincident or parallel with a normal vector on the calculated least squares approximate surface, to the acquisition attitude at the next acquisition position.
By thus determining the acquisition attitude at the next acquisition position, it is possible to calculate a normal vector which represents a perpendicular direction to a plane in the acquisition area with higher accuracy without being adversely affected by uneven high-frequency components which might be contained in measured data acquired from the acquisition area. In other words, it is possible to further increase the parallelism of the reference surface of the interferometer in the measuring means with a corresponding acquisition area on the surface to be inspected.
Preferably, after the measuring means has been positioned to the next acquisition position, the control means calculates a number of optical interference fringes indicated by measured image data acquired at the next acquisition position, based on the measured image data, and sets an acquisition attitude in which the number of optical interference fringes becomes equal to or smaller than a predetermined number, to the acquisition attitude at the next acquisition position.
By thus determining the acquisition attitude at the next acquisition position, it is possible to obtain an appropriate acquisition attitude to be taken at the next acquisition position.
Preferably, the surface shape analyzing and calculating means has an intermediate-area surface shape calculation function of calculating a surface shape of an overlapping area portion where two adjacent ones of acquisition areas corresponding to measured image data acquired at respective acquisition positions overlap, and the intermediate-area surface shape calculation function comprises synthesizing surface shapes of the two adjacent ones of the acquisition areas, to thereby calculate the surface shape of the overlapping area portion.
By thus calculating the surface shape of the overlapping area portion, from the result of measurement of the surface to be inspected provides, it is possible to obtain a surface shape of the overlapping area portion of two adjacent acquisition areas, which is smooth and has no discontinuity.
More preferably, the overlapping area portion whose surface shape is calculated by the intermediate-area surface shape calculation function exists between a first position corresponding to an acquisition position of one of the two adjacent ones of the acquisition areas and a second position corresponding to an acquisition position of another of the two adjacent ones of the acquisition areas, and the intermediate-area surface shape calculation function comprises calculating a surface shape at an object position between the first position in the one acquisition area and the second position in the another acquisition area, by synthesizing a surface shape of the one acquisition area and a surface shape of the another acquisition area by using a weighting factor defined by a ratio between a distance from the first position in the one acquisition area to the object position and a distance from the object position to the second position in the another acquisition area.
By thus calculating the surface shape of the overlapping area portion, it is possible to determine a surface shape in the overlapping area portion of two adjacent acquisition areas, which is smooth and has no discontinuity, and facilitate the calculation of the surface shape.
The above and other objects of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.